Multifocal Osteoblastoma of the Jaws: A Very Rare Case Report

Osteoblastoma is a solitary benign bone-forming neoplasm, which comprises 1% of all primary bone tumors. Multifocal benign osteoblastoma of the jaws is very rare. Osteoblastoma must be differentiated from other similar bone-forming lesions such as osteoid osteoma and osteosarcoma for correct diagnosis and proper treatment planning. Therefore, precise examination of the patient and correlation with radiographic and histological features are essential for the best treatment and prognosis. This study reports a rare case of multifocal osteoblastoma in a 30-year-old female, involving the mandible and the maxilla, which was treated by surgical excision, iliac bone graft reconstruction, and implantation. Complete surgical excision is necessary to treat osteoblastoma with a good prognosis. The patient was followed-up for four years postoperatively, and there were no signs of recurrence in the panoramic view or the clinical examination


Introduction
Osteoblastoma (OB) is a solitary benign tumor that was first described by Jaffe and Lichtenstein in 1956 [1][2].
This benign bone-forming neoplasm is uncommon and comprises 1% of all primary bone tumors and only 3.5% of all benign bone tumors [6][7]. It is most commonly found in the vertebral column and the long bones. About 10%-12% of OBs occur within the gnathic bone with the mandibular posterior region being affected 2-3 times more frequently than the maxilla [2,[4][5][6].
Patients often present with a painful mass not relieved by the non-steroidal anti-inflammatory drugs (NSAIDs) and with clinical signs related to swelling, obstruction, tooth mobility, tooth displacement, root resorption or compression of the surrounding structures [3,[8][9].
Rarely, these tumors may exhibit progressive growth, local recurrence, and malignant transformation [2]. Aggressive osteoblastoma (AO) is the type of osteoblastoma, which is clinically and radiographically larger (˃4 cm) than conventional OB [10]. AO affects an older age group and pain is common. The probability of recurrence is 13.6% for conventional OB and 50% for AO [11].
OB is a rare bone tumor, which is usually seen as solitary. In very rare cases, it may occur as an isolated focus in more than one bone or as multiple foci within one or more bones [10,12].
To date, only one case of multifocal OB of the maxilla and the mandible has been reported [13].

Case Presentation
A 30-year-old woman with mild pain and swelling on the right side of the mandible since two months ago referred to the dentist. She had taken NSAID to relief the mild pain. Panoramic radiography was taken. The lower right first molar was extracted due to luxation. No other treatment was performed. She was referred to an oral and maxillofacial surgeon. There was no history of trauma to the maxillofacial region. The patient had a history of hypothyroidism for the last seven years and was taking levothyroxine (10 mg per day).
In the clinical examination, mild extraoral swelling on the buccal aspect of the right body of the mandible and intraoral moderate swelling from the right first premolar to the right retromolar region of the mandible were found. The clinical differential diagnoses of this lesion were residual cyst, keratocystic odontogenic tumor (KOT), ameloblastoma, and odontogenic myxoma.    Then, the pre-bent reconstruction plate was fixed intraorally to the mandible using six titanium screws. The inferior alveolar nerve was preserved with no injury ( Figure 5). There were no clinical or radiographic signs of recurrence at the one-, three-, six-months and one-, two-, three-, four-years follow-ups ( Figure 6).
After six months, the reconstruction plate was removed intraorally, and the hard tissue was reconstructed using an iliac bone graft under general anesthesia (Figure 7). The inferior alveolar nerve was repositioned laterally. Then, the iliac bone graft was fixed with four screws (Figure 8). There was no histological evidence of recurrence. Four months later, implantation of the right mandibular body was performed under local anesthesia ( Figure 9).
Yu et al. [12] described a case of an 8-year-old girl who was diagnosed with the Cushing syndrome and multiple OB in the femur, the pelvis, and the skull base.
Kyriakos et al. [14] presented two adult patients with multifocal osteolytic lesions, radiographically simulating a vascular tumor. Multiple bones have been involved in one patient [20]. Adler [15] reported a 9-year-old child with multiple OB in multiple bones of the right hand.
To Date, there is only one report of multiple lesions in the jaws. Asada et al. [13] described a case of OBs at multiple sites of the maxilla and the mandible, one of which was accompanied by simple bone cysts. They presented a 38-year-old woman with painless, non-growing swelling on the right side of the mandible for three years. The tumors produced hazy or ground-glass-like  There was no sign of recurrence of the lesions one year and six months postoperatively [13].
Radiological manifestations of OB are variable and nonspecific but usually indicate a lesion that is generally greater than 2cm, oval, or round, expansile, well circumscribed, and predominantly lytic with a rim of reactive sclerosis. The central portion can be completely lytic but usually shows calcifications, at least focally [16].
In the present case, the lesions showed well-defined, partially corticated, and expansile borders without a scl-  Considering the rarity of multifocal benign OB of the jaws, there may be some differential diagnoses such as fibro-osseous lesions, osteoid osteoma (OO), osteosarcoma, cementoblastoma, and ossifying fibroma.
OB and OO are closely related benign bone tumors.
Indeed, OO is the main differential diagnosis challenge of our case. The OO exhibits more limited growth potential than OB (less than 1.5-2cm). The pain of OO is relieved by the NSAIDs. Radiographically, OO manifests an intracortical nidus with different amounts of mineralization, cortical thickening, sclerosis, and bone marrow edema [17]. The surrounding reactive sclerosis is less prominent in OB than in OO. The lesions in the present case were differentiated from OO because of the tumor size on the right side of the mandible and the absence of a central nidus of osteoid tissue.
Histologically, OB is composed of haphazardly deposited trabeculae or sheets of woven bone and osteoid rimmed by osteoblasts and scattered osteoclasts that are enmeshed in a richly vascular stroma [18][19]. In the pr- The pathology report indicated benign OB in the maxilla and the mandible. Right mandible lesion shows more epithelioid osteoblastoma cells so the diagnosis for this lesion is AO; therefore, it was named multifocal OB of the jaws. Incidentally, we can designate the lesion located on the right side of the mandible as AO, concerning its size and microscopic features. The clinical and radiographical appearance of OB is very similar to peripheral ossifying fibroma or any fibro-osseous lesion. The central histological feature to differentiate this pattern from fibro-osseous lesions is that the stroma does not consist of cellular spindle cells but rather a loose vascular stroma with numerous prominent epithelioid-type osteoblasts [21].
The common approach to treating OB is resection with a 5mm safety margin [2,7]. The alternative therapies of enucleation, curettage, and peripheral osteotomy are also valid but have specifically limited indications [4,20]. Kyriakos et al. [14] described two adult patients with multifocal osteolytic lesions in the leg, which were removed by en bloc excision. Mahajan et al. [4] described a 45-year-old female with swelling on the right lower back since five years ago, which was removed through excision.
Regarding the present case, according to the extension of the lesion and the histopathological findings, conservative treatment was chosen, including enucleation, curettage, and peripheral intraoral ostectomy. The inferior alveolar nerve was preserved and displaced laterally.
There were no clinical or radiographic signs of recurrence at the one-, three-, six-months and one-, two-, three-, four-year follow-ups.
Written informed consent was obtained from patient prior to this case presentation.

Conclusion
Multifocal benign OB of the jaws is a very rare bone tumor. This tumor must be differentiated from other similar bone-forming lesions such as OO and osteosarcoma for correct diagnosis and proper treatment planning. Therefore, precise examination of the patient and correlation with radiographic and histological features are essential for the best treatment and prognosis. Complete surgical excision is necessary to treat OB with a good prognosis.